Genesis and accumulation mechanism of low rank coalbed methane in gas-rich depressions of Erlian Basin

Inner Mongolia is rich in low rank coalbed methane (CBM) resources, and research on the genetic and accumulation mechanism of coalbed methane is crucial for the selection and evaluation of low rank coalbed methane resources. Taking the low rank CBM in the key gas rich depressions of the Erlian Basin as the research objective, the geochemical characteristics of CBM and coal seam water is analyzed and the genesis and accumulation mechanism of low rank CBM is revealed by using various experimental methods, including tests on composition and carbon/hydrogen isotopes of CBM water quality, hydrogen/oxygen isotopes, and radioactive isotopes ³H and ¹⁴C of coal seam water. The result shows that the CBM component in the Erlian Basin is dominated by CH₄, which belongs to dry gas. The CH₄ volume fraction increases with the increase of burial depth, while the CO₂ volume fraction first increases and then decreases with the increase of burial depth, and it shows high values between 300 and 500 m. The carbon and hydrogen isotopes of CH₄ in the research area are generally lighter, with and δ¹³C(CH₄) ranges from -70.3‰ to -48.0‰ and δD(CH₄) ranges from -285.5‰ to -189.0‰, and δ¹³C(CO₂) varies between -37.6‰ and 1.94‰. The chemical types of coal seam water are mainly HCO₃-Na type and Cl·HCO₃-Na type, and the present water environment of coal seam is relatively stable, with weak hydrodynamic forces. The apparent age of coal seam water is about 1 020-47 490 years, mainly sourced from Quaternary atmospheric precipitation, and there is no or less current surface water supply. The CBM in the Erlian Basin is mainly primary biogenic gas, mixed with a small amount of early thermogenic gas. As the burial depth increases, the geological environment and types of methanogenic archaea change, and the production pathway of biogenic methane also changes. In the Jirgalangtu depression, the biogenic methane is mainly produced by acetic acid fermentation pathway in the early stage, and is converted to CO₂ reduction pathway in the late stage, mixed with a small amount of low mature thermogenic gas. In both Bayanhua and Huolinhe depressions, acetic acid fermentation is the main pathway of biogenic gas production, and there is also a small amount of methyl fermentation biogenic gas in Huolinhe depression. The research area has the "three low" coal seam conditions suitable for biogenetic gas generation, including low geothermal temperature, low salinity, and low thermal evolution. The Jirgalangtu depression belongs to the graben type biogenetic gas accumulation model in shallow thick coal seam, while the Bayanhua and Huolinhe depressions belong to the semi-graben type biogenetic gas accumulation model in middle-deep confined areas with hydraulic sealing effect. Finding favorable target areas for the formation and enrichment of biogenic gas should be a key direction for the future exploration and development of CBM in the Erlian Basin, and it is also a practical guarantee for increasing the storage and production of low rank CBM in the Erlian Basin.